Method of making precision castings



Jan. 8, 1946.

s'. M. STOODY ET AL METHOD OF MAKING PRECISION CASTINGS Filed Dec. 24,1941 2 Sheets-Sheet l R. m. m N Y B w 5 5 my mm A z Arrom/svs.

Jan. 8, 1946. s. M. STOODY ET AL METHOD OF MAKING PRECISION CASTINGSFiled Dec. 24, 1941 2 Sheets-Sheet 2 u 1 I n I I I. 1 1 I I 1 I I 1 1 1n I 5HLL yM. Sroom; RALPH 4505,

M m mwm Y B ATTORNEYS.

Patented Jan. 8, 1946 METHOD or MAKING PRECISION CASTINGS Shelley M.Stoody and Ralph L. Abos, Whittier, Calif., assignors to Stoody Company,Whittier, Calif., a corporation of California.

Application December 24, 1941, Serial No. 424,370

3 Claims.

This invention relates to a method of making precision castings.

A primary object of the invention is to provide a method whereby amultiplicity of castings may be produced which is perfectly formed so as.to obviate the necessity of finish machining them, the

castings being characterized by the fact that they are formed of acorrosion-resistant strong,

high melting point metal which, because of its bine having blades drivenby the exhaust gasesof an internal combustion engine usually an airplanemotor. The rotor carrying these blades is driven at high speeds usuallyin the neighborhood of 20,000 revolutions per minute. The blade on therotor, which are subject to the direct action of the exhaust gases fromthe airplane engine, are heated to a temperature in the neighborhood of1200 F. The metal, forming the blades must be resistant to the burningaction of the exhaust gases under these high temperatures and yet mustbe of great tensile strength to resist centrifugal forces developedtherein under the high rotary speeds. Normally such metal should becapable of possessing a tensile strength of approximately 70,000 poundsper square inch at a temperature of 1200 F.

Metals that possess the desirable characteristics usually containvarious percentages of cobalt, chromium, and tungsten. Some of themetals sold under the trade-name of Stellite, Vitalium, and Stoodite aresuitable. These metals, however, have the common characteristics of highmelting point and the fact that they are inherently hard, beingconsiderably harder than ordinary steel. For this reason, they are verydifficult to machine, and when the casting is in the form of anirregular shape, s in the case of a turbine blade, its shape togetherwith the extreme hardness of the metal makes the machining operation ofeach blade extremely expensive. The purpose of the present invention isto devise a method wherein these high melting point, inherently hardmaterials can be very accurately cast to the exact 5 shape desired sothat finish machining can be practically entirely avoided, thus reducingthe labor cost on the construction of the blades. Manifestly, with arotor turning at high speed th blades must be not only accurately formedbut uniform for the reason that only small differences 7 in the weightsof the blades will throw the entire rotor out of balance under itsrunning conditions. It will therefore be understood that the presentmethod contemplates the manufacturing of a multiplicity of accuratelyformed castings wherein, finish machine work is reduced to a very smallminimum and characterized by the fact that a high melting point,inherently hard metal is used which in the casting is of uniform densityand homogeneous throughout.

With the foregoing and other objects in view, which will be mademanifest in the following detailed description and specifically pointedout in the appended claims, reference is had to the accompanyingdrawings for an illustrative embodiment of the invention, wherein:

Figure l is a sectional view through a die casting apparatusillustrating the die in position therein preparatory to the die castingof the pattern;

Fig. 2 is a view similar to Fig. 1 illustrating the pattern as havingbeen die cast;

Fig. 3 is a sectional view through a carbon flask illustrating the diecast pattern in position therein, preparatory to rammin up thecarbonaceous mold;

Fig. 4 is a sectional view through the flask illustrating thecarbonaceous mold as having been rammed up about the pattern;

Fig. 5 is a view similar to Fig. 4, but illustrating the die castpattern as having been mailed out or removed from the mold;

Fig. 6 is a vertical section through a centrifugal casting apparatusillustrating the molds as having been filled 'by the cast metal; and

Fig. '7 is a perspective view of the casting produced.

Referring to the accompanying drawings wherein similar referencecharacters designate similar parts throughout, and referring first toFigs. 1

and 2, inasmuch as each of the multiplicity of rotor blades that is tobe produced is an exact duplicate of each other. it is desirable toproduce a multiplicity of patterns all of which will be exact duplicatesof each other. To this end the patterns are die cast. The die isindicated as having been made up oftwo opposed parts in and I l and inthe opposed faces of these parts there is formed the mold cavity 12which is of the exact size and shape of the finished rotor blade orcasting that is to be produced allowance being made merely for theshrinkage involved in the casting of the metal that finally forms thefinished product.

Below the mold cavity l2 there is a small sprue l3 which is designed toform the mold entrance and at the top of the die there is-an extensionof the mold cavity 1 i, the extension being indicated at 13 whichprovides a vent for the mold. Die casting metal I! is heated in asuitable container il having at its bottom a plunger is that can belifted bya lever II. This die casting metal is heated and melted by oneor more burners 20. On the top of the die there is a cover 2i having acavity 22 which receives excess metal from the die as shown in Fig. 2.The metal that we preierably employ for the pattern is a die castingmetal known in the trade as Rose metal. It comprises approximately 50%bismuth, 28% lead, and 22% tin. It melts at approximately 212 F. and hasthe characteristic that on solidfying it neither expands or contractsmaterially.

The first step of the present method consists of die casting the patternwherein the molten metal I! is forced up through the die by the pistonor plunger l3 filling the die with the excess metal entering cavity 22as shown in Fig. 2. By die casting the pattern it is manifest that amultiplicity of patternscan be produced, all of which will be exactduplicates of each other. One of such patterns is indicated at 23 which,after having the excess metal at the top and bottom trimmed off, ispositioned in a carbon-flask 24. While other materials might be suitablefor use as a flask graphitic carbon is preferred. Into the flask isplaced a carbonaceous molding material 25.

A feature of the invention resides in the use of the correct compositionof molding material. The

preferred composition for the mold consists of the following ingredientsin the following proportions;

Ounces Per cent OO NGUG that is sold under the trade-name of Hi-Link.

70 chilled by the cold walls of these passages and The fire clay servessomewhat as a binder for the carbon particles. The glucose and boiledlinseed oil also serve somewhat as a binder and carburizer during thebaking process subsequently described. The water is added to moisten the55 of the passages.

trace of aerosol," a dicctyl ester of sodium sulpho-succinate. Thismaterial is usually" obtainablein a aqueous cloudy solution and a traceis added to the water so that the boiled linseed oil will quickly forman emulsion with the water. The aerosol acts as a very good emulsifyingagent for this purpose. These ingredients are thor: oughly mixed and arerammed or tamped in the flask 23 around the pattern. Some degree ofskill .10 is required in order to properly ram up the mold,

and the best way to describe the ramming of the mold is to state thatapproximately a fifty pound tamp is used.

When the mold is completely rammed up about the pattern as shown in Fig.4. the fiask and the continuous carbonaceous material are then placed 1in a heated oven. Itis first heated to a temperature of approximately150 for twelve hours during which period the mold is thoroughly dried.

0 Following the twelve hour drying the mold is then temperature is wellin excess of the melting point of the die cast metal forming thepattern. The pattern-consequently quickly melts and runs out either orboth ends of the mold. It is preferable to wash the mold atthe end ofthe baking period. This can be accomplished by dipping the mold in hotmercury which has the property'of dissolving or amalsamating with anysmall particles of die casting metal that may remain in comers of themold. The mercury may be heated from 300 to 350 F. The use .of mercuryis some- 40 completed the mold will be found to be reasonably hard andit is then-in condition to be used in centrifusally casting the finishedcasting.

As a means for centrifugally casting a casting machine is illustrated inFig. 6 wherein a motor 28 rotates a carrier 21 about a vertical axis.This carrier receives upper and lower mold parts 28 and 28. In the uppermold part thereis formed a plurality of cylindrical apertures 30 each ofwhich receives a flask 24 and its contained mold. 1 The upper part 28has a central gate 3| which communicates with the inner ends 01' radialpassages 32 formed in the lower part 29. The mold entrances communicatewith these radial passages at points 33 disposed inwardly of the outerends The vents from the mold are open at the top thereof and any excessmetal forced out of the vents is confined by a suitable cover 34.

The alloy that is to be cast is poured in molten so condition into thecentral gate 3| and fiows outwardly into the passages while the mold isbeing rotated by the motor 26. In making castings of this characterusing the metal hereafter described we prefer to rotate the mold atapproximately 1500 R. P. M. with the centers of the molds radiallyspaced approximately 4" from the center of rotation. That metal whichfirst enters the gate 3i and the inner ends of passages 32 may beregarded as initial metal. This metal is somewhat mass and it ispreferable to add to the water a does not enter the mold but instead isdrawn out the trailing metal is caused to enter the mold v cavities withthe result that the casting produced will be of fine grain andhomogeneous being characterized by an entire absence of blow holes orgas pockets. Sufiicient metal is poured into the gate 3| to fill all ofthe mold cavities. Usually there is a small excess which runs out thetop through the vents.

The metal that we prefer to employ for the purpose of manufacturingrotor blades of the Moss supercharger is the metal known in the trade asStoodite #6. It comprises approximately 69% cobalt, 25% chromium, 5%tungsten carbide (W3C) and 1% of 85% ferrosilicon. This metal is highlyresistant to the effects of exhaust gases from an internal combustionengine even when the metal is subjected to a 1200 temperature. It hasthe required tensile strength to resist separation under the highcentrifugal forces that are developed therein when the metal is servingas a rotor blade of the supercharger. Such a metal has a relatively highmelting point and is inherently hard. Because of its high melting pointa carbonaceous mold material is practically essential to the presentprocess. Other mold materials, such as for example those'materials usedfor making investments in dental casting, are incapable of serving asmold materials for the present process. These investment materials breakdown under the high temperature of the metal. A carbonaceous moldingmaterial such as that herein described will stand up regardless of thehigh temperature. v

The mold material used in the present process must not only be capableof standing up under high temperature but it also must be sufllcientlyhard so that the metal rushing into the mold will not wash away cornersof the mold cavity or cause small projections in the mold cavity tobreak oil. Although the mold material must be sufliciently hard toresist damage by the metal at the same time after the baking operationit must still be sufficiently soft or friable so as to be capable ofgiving or crushing slightly when the casting contracts on solidifyingand cooling. If the mold is extremely hard, then the casting gavereasonably satisfactory results, but'was sub- :Iect to the criticismthat the mold was slightly too soft, resulting in the mold being damagedby the metal rushing into it. Mold materials having the followingingredients gave reasonably satisfactory results:

Ownecl Permit Ouncel Percent Carbon 4% 73.5 Fire clay M 4.1 Glucose 116.3 Boiled linseed oil 54 4. 1 Water M 2.0

The requirements of the mold material will vary somewhat depending uponthe type of metal used and casting conditions, and to some extent therequirements of the mold material will depend upon the size and shape ofthe casting that is to be produced. We have found with our moldingmaterial that the structure and the hardness of the mold can to someextent be controlled by the amount of glucose, boiled linseed oil, andwater added, the hardness increasing mainly with the addition of waterand glucose. We have proposed substituting for the glucose caramel, but

- prefer ,the glucose due to the fact that it is cheapon contractingwill be held against contraction Ounces Percent Carbon.. 4, 5 78.0Fireclay M 4.4 Glucose A 13.2 Boiled linseed oil M 4.4

er, seems to be more effective, and produces a molding material thatseems to be easier to work.

We have also proposed using dextrln in place of glucose but found thatglucose gives superior resuits. In place of the fire clay we haveproposed using Portland cement as a binder for the carbon but find thatwhen the mold material is made in this manner that the surface of themold seems to break down and the hot metal tends to cut or eat into thesurface of the mold. We have also proposed using pitch but find thatthis is objectionable in that the pitch tends to burn the surface .ofthe mold, tends to break down under the high temperature'of the metal,and a mold that is too hard is normally produced. If the mold is bakedat too high a temperature constituents of the pitch also volatilize. Wehave found, however, that a natural yellow clay obtainable near SantaAna, California, may be substituted for fire clay,

and produce reasonably satisfactory results. Care should be taken tohave the carbon finely ground that is capable of passing through a 200mesh screen. If the carbon particles are of larger size we find that weare unable to get the same binding action in makingthe mold and the wallsurfaces of the mold cavity are afiected'by being quite rough.

By carefully following the preferred steps of the method as aboveoutlined, we are able to obtain an intricate casting of the characterillustrated in Fig. 7, which is perfectly formed, having smooth surfacesrequiring no other machining than the grinding off of the metal in themold entrance and the metal in the vent. As the patterns are made by diecasting they may be very accurately produced with the result that amultiplicity of rotor blades can be produced which will be of exactlythe same size, shape, and weight. This ability to produce exactlyuniform castings is of equal importance to the avoidance of machine workin constructing a turbine rotor for a supercharger. It will be obvious,however, that the present method may be utilized whenever it is desiredto employ a strong metal and avoid machine work. 4

Themethod or centrifugal casting is not claimed application which wehave flied contemporaneously herewith, Serial No. 424,371 flied December24, 1941. Also, the molding composition is not claimed herein but formsthe subject matter of another application which we have filedcontemporaneously herewith Serial No. 424.369 filed December 24, 1941.

Various changes may be made in the details of construction withoutdeparting from the spirit or scope of theinvention as defined by theappended claims.

We claim:

1. The method of making castings oi cobaltchromium tungsten alloys whichincludes making a diecast pattern, ramming up a carbonaceous herein butforms the subject matter of another mold composed principally of finelydivided carbon and a binder about the pattern, baking the mold, meltingout the pattern and centriiugaliy casting molten cobalt-chromiumtungsten alloyinto' the mold.

2'. The methodoi making castings of cobaltchromium'tungsten alloys whichincludes making a; diecast pattern. ramming up a carbonaceous moldcomposed principally of finely divided carbon and a binderaboutthepattern, baking the mold, melting out the pattern and centrifugally thepattern, and centrii'ugally casting molten cobalt-chromium tungstenalloy into the mold.

. SHELLEY M. STOODY.

RALPH L. ABOS.

